Hitherto, as a process for producing a sulfur-containing hydroxycarboxylic acid, hydrolysis of a hydroxynitrile compound (cyanhydrin) has been employed. Industrially, sulfuric acid is used as a catalyst. Further, for example, JP 58-15120 B, JP 2-84198 A and JP 4-40898 disclose hydrolysis of a hydroxynitrile compound by the action of a microorganism to convert it into a corresponding hydroxycarboxylic acid.
However, in a process using sulfuric acid as a catalyst, a hydroxynitrile compound is reacted with sulfuric acid to produce, in addition to the objective hydroxycarboxylic acid, an equimolar amount of ammonium sulfate as a by-product. Therefore, a step for recovering the by-product is required, which makes production steps complicated. Further, in a process for producing a hydroxycarboxylic acid compound from a corresponding hydroxynitrile compound by using a microorganism, there are such problems that the enzyme activity possessed by the microorganism is inhibited by degradation products from the hydroxynitrile compound, i.e., cyan, etc., and treatment of a large amount of an ammonium salt produced is required, thereby accompanying an increase in the production cost.